首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   10741篇
  免费   498篇
  2020年   165篇
  2019年   238篇
  2018年   306篇
  2017年   296篇
  2016年   372篇
  2015年   239篇
  2014年   241篇
  2013年   3032篇
  2012年   317篇
  2011年   366篇
  2010年   358篇
  2009年   311篇
  2008年   266篇
  2007年   284篇
  2006年   245篇
  2005年   159篇
  2004年   135篇
  2003年   143篇
  2002年   152篇
  2001年   193篇
  2000年   156篇
  1999年   176篇
  1998年   131篇
  1997年   109篇
  1996年   107篇
  1995年   107篇
  1994年   84篇
  1993年   110篇
  1992年   91篇
  1991年   89篇
  1990年   95篇
  1989年   88篇
  1988年   79篇
  1987年   99篇
  1986年   94篇
  1985年   108篇
  1984年   124篇
  1983年   123篇
  1982年   99篇
  1981年   107篇
  1980年   103篇
  1979年   99篇
  1978年   99篇
  1977年   96篇
  1976年   84篇
  1975年   90篇
  1974年   66篇
  1973年   63篇
  1972年   65篇
  1970年   55篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
101.
A new and recursive algorithm for determining proximal polygons is based on quadtree concepts. The algorithm can use any distance metric, and produces a quadtree of the image of the proximal polygons. This can be converted to vector form, or be incorporated directly into a quadtree-based geographic information system, in order to solve a number of closest-point problems. The algorithm can also produce the Delaunay triangulation, which is the dual of the proximal polygons. Empirically, the running time of the algorithm is proportional to the number of centers raised to the 1.6 power.  相似文献   
102.
243 blacksmiths in Sierra Leone were interviewed in 1984-85 in an effort to focus on both the activities and attitudes of traditional blacksmiths in the country's economy. Due to the fact of an shortage of foreign exchange with which to import and maintain equipment, agriculture using high-technology equipment accounts for less than 15% of total production. Consequently, blacksmiths are vital to the nations' survival, despite prevailing attitudes toward them. The interviews were conducted in 9 of the 12 districts in Sierra Leone. The blacksmith operates not simply in terms of producing and servicing goods; cultural values frame his position in the community. In all of West Africa, and particularly among Mende-related peoples, there historically exists a mystique surrounding a blacksmith. In some societies, blacksmiths were believed to be witches. The arguments that most likely could account for this would probably lie in the fact that the blacksmith made farm tools and weapons of war, means of survival in the community. Thus, his position was vital. Traditionally the importance of the smith's profession lies in the fact that some of the implements he fabricates and the materials he uses are believed to provide elements of social control or to have healing powers. An appreciation of the cultural significance of the blacksmith demonstrates the degree of attachment of the population to this profession as well as the context within which one could relate to possible technological changes in the trade. An attempt was made in the interview to gather some information about levels of production. The figures represent averages, and production capacity varied owing to a number of factors, including the degree of organization of the unit, the capacity of the head of the forge to keep his team busy when work was intensive, and the degree of energy and determination of an operator to get the greatest amount of work done in 1 day. The most obvious factor influencing production levels was the size of the units. Pricing of commodities and the blacksmith's services depended on a combination of factors, but, generally, in semi-urban areas, where more impersonal relationships tended to obtain, prices appeared relatively more fixed than in rural areas. All smiths interviewed complained about the increasing scarcity of scrap iron. As there is every reason to envisage a continued dependence on informal-sector production by blacksmiths, official attitudes need to take these trades more seriously.  相似文献   
103.
104.
105.
106.
George Woodcock was anarchism's most influential historian and an important public intellectual in Canada. This article focuses on his engagement with Canadian nationalism in the 1960s and 1970s. It argues that a ‘philosophical anarchism’ was at the heart of his intellectual project, and this informed his reading of Canadian cultural development and subsequent political challenge to Pierre Elliott Trudeau's civic nationalism. Woodcock decoupled the concepts of ‘nation’ and ‘state’ in order to develop a radically different model for Canada—the ‘anti-nation’—defined by regionalism, federalism and direct democracy. His reading of Canada's cultural history supporting this position was therefore part of a strategy to repurpose nationalist rhetoric towards anti-state ends.  相似文献   
107.
Biogeographers, ecologists, palaeontologists, and conservation managers often deal with checklists in which not all individuals have been identified to a species level, or the accuracy of species identification is questionable. Is it possible and credible to investigate species richness based on such checklists? Studies on macrofauna in the Far Eastern seas, eastern Arctic seas, and adjacent waters of the Pacific and Arctic Oceans suggest that in different habitats and for diverse taxa, species, and higher taxa richness strongly correlate with each other and increase with an expansion in the study area and sample size according to the species–area law. Such an increase is higher in the bottom zone than in the pelagic. Species and higher taxa richness also show a decrease from lower to higher latitudes, which is in line with the Humboldt–Wallace’s law. According to Willis’ law and self-similarity in the organisation of taxonomic levels, species richness can be assessed based on the genus, family, and order richness. In other words, supraspecies richness itself can tell us the same as species richness and therefore certain global patterns revealed at the species level may also be revealed at the supraspecies level. Such a concordance in general trends among richness parameters at different taxonomic levels in practice implies that species richness can be studied based on lists that lack species identifications or lists with doubtful species identification. We suggest bolder use of supraspecies richness in science and practice, discussing the disadvantages and advantages of this approach.  相似文献   
108.
This paper is a statistical and geological study of the results of analysis of flint from British and West European flint mines, carried out by emission and atomic absorption spectroscopy. Groups of flint specimens were studied from each geographically separate flint mining area. Flints were analysed for a group of easily measured trace elements shown by previous work to be generally present. The ratios between the trace elements form a consistent pattern for each flint mine and statistically valid differences of pattern can be recognized between different flint mines. A geological investigation shows that the measured trace elements may be derived from clay minerals and explains geographical and statistical variation in the composition of flint in terms of its mode of origin in the chalk. An advanced statistical technique allows individual flint specimens of unknown origin to be attributed by their composition to one or other of the identified flint sources. The archaeological implications of this study are discussed.  相似文献   
109.
110.
A method is described for the identification of the origin of flint artefacts. The method is based on the variation in the trace element concentrations for different origins. The trace element concentrations were determined by non-destructive neutron activation analysis. The identification was obtained from a pattern recognition analysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号